1. Field of the Invention
This invention relates to a valve timing control system for an internal combustion engine, which varies the cam phase of at least one of an intake cam and an exhaust cam, relative to a crankshaft of the engine, to thereby control valve timing, and more particularly to a valve timing control system which is used together with a deterioration-determining system for performing degradation determination of at least one of an oxygen concentration sensor and an exhaust gas-purifying device.
2. Description of the Prior Art
Conventionally, a valve timing control system of the above-mentioned kind was proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 9-217609. In this control system, a cam phase change mechanism supplied with hydraulic pressure controlled by an hydraulic pressure control valve changes the cam phase by changing the angle of a camshaft relative to a cam pulley. Further, a target cam phase is set depending on operating conditions of an internal combustion engine, e.g. on rotational speed of the engine and load thereon, and at the same time, the control system controls the cam phase such that an actual cam phase becomes equal to the target cam phase. This varies a valve overlap between an intake valve and an exhaust valve, thereby ensuring charging efficiency and internal EGR suitable for the operating conditions of the engine.
Further, some of the recent internal combustion engines include a degradation-determining system for performing degradation determination of an oxygen concentration sensor or an exhaust gas-purifying device. For example, Japanese Laid-Open Patent Publication (Kokai) No. 8-121221 discloses one of such conventional degradation-determining systems for an oxygen concentration sensor. This oxygen concentration sensor is arranged in an exhaust manifold connected to cylinders, and the degradation-determining system sets fuel injection amounts for the cylinders to respective fixed values such that the air-fuel ratio of a mixture supplied to at least one of the cylinders is different from those of mixtures supplied to the other cylinders, over a predetermined time period, in a steady operating condition of the engine in which the engine rotational speed and the intake air amount are constant. A waveform variation of the air-fuel ratio and a waveform variation of output from the oxygen concentration sensor caused by those fuel injection amounts during the predetermined time period are compared with each other, whereby the degradation of the oxygen concentration sensor is determined.
Further, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2000-328929 discloses one of such degradation-determining systems for an exhaust gas-purifying device. This exhaust gas-purifying device is comprised of a linear-type oxygen concentration sensor, a three-way catalyst, a first oxygen concentration sensor of a binary type, a NOx-reducing device, and a second oxygen concentration sensor of a binary type, which are sequentially arranged in an exhaust system of an internal combustion engine from upstream to downstream. This degradation-determining system determines degradation of the NOx-reducing device based on a time period from a time point the output from the first oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio after the mixture supplied to the engine was enriched, to a time point the output from the second oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio, and a degree of degradation of the three-way catalyst.
However, when the conventional valve timing control system and the degradation-determining system are employed in combination, the accuracy of the degradation determination is lowered. More specifically, if the target cam phase is changed depending on the rotational speed of the engine or load thereon, the actual cam phase is changed in accordance therewith, so that the valve overlap is changed. As a result, the charging efficiency is changed to change the intake air amount, which results in variation in the air-fuel ratio of the mixture supplied to the engine. Further, the cam phase is changed by the cam phase change mechanism, which is hydraulically controlled, and hence it takes time to converge the actual cam phase to the target cam phase, so that the air-fuel ratio varies or fluctuates over a long time period. On the other hand, the two conventional degradation-determining systems described above are both configured such that the degradation determination is carried out under the condition of the constant intake air amount and in the state of the air-fuel ratio being controlled to the predetermined ratio. Therefore, when the valve timing control and the degradation determination are carried out in parallel, the accuracy of thereof is lowered due to variation in the air-fuel ratio caused by changes in the intake air amount during execution of the degradation determination.
It is an object of the invention to provide a valve timing control system for an internal combustion engine, which is capable of properly controlling valve timing while maintaining the accuracy of degradation determination of an oxygen concentration sensor or an exhaust gas-purifying device.
To attain the above object, the present invention provides a valve timing control system for an internal combustion engine, for controlling valve timing by changing a cam phase which is a phase of at least one of an intake cam and an exhaust cam, relative to a crankshaft, the valve timing control system being used together with a degradation-determining system for performing degradation determination of at least one of an oxygen concentration sensor and an exhaust gas-purifying device each arranged in an exhaust system of the engine, in a state in which an air-fuel ratio of a mixture supplied to the engine is controlled to a predetermined state.
The valve timing control system according to the invention is characterized by comprising:
actual cam phase-detecting means for detecting an actual cam phase;
operating condition-detecting means for detecting operating conditions of the engine;
target cam phase-setting means for setting a target cam phase in dependence on the detected operating conditions of the engine;
control means for performing control such that the actual cam phase becomes equal to the target cam phase;
determination means for determining whether or not executing conditions for executing the degradation determination are satisfied; and
target cam phase-restricting means for restricting a change in setting of the target cam phase in dependence on the detected operating conditions of the engine by the target cam phase-setting means, when the determination means has determined that the executing conditions for executing the degradation determination are satisfied.
According to this valve timing control system, the target cam phase is set in dependence on the operating conditions of the engine, and the actual cam phase is controlled to become equal to the target cam phase. Further, when it is determined that executing conditions for executing the degradation determination of the oxygen concentration sensor and/or the exhaust gas-purifying device by the degradation-determining system are satisfied, a change in setting of the target cam phase in dependence on the operating conditions of the engine is restricted. This suppresses the variation in the actual cam phase, and accordingly suppresses changes in the valve overlap and charging efficiency, whereby the changes in the intake air amount and resulting variation the air-fuel ratio can be suppressed. As a result, during execution of the degradation determination, the air-fuel ratio can be maintained in a predetermined state absolutely necessary for the accurate degradation determination, and therefore the degradation determination of the oxygen concentration sensor or the exhaust gas-purifying device can be carried out with accuracy.
Preferably, the target cam phase-restricting means sets the target cam phase to a fixed value.
According to this preferred embodiment, when it is determined that the executing conditions are satisfied, the target cam phase is set to a fixed value. This reduces variation in the actual cam phase, which sufficiently suppresses the variation in the air-fuel ratio. Therefore, the degradation determination can be carried out with high accuracy.
More preferably, the valve timing control system further includes a hydraulic pressure control valve that holds the cam phase in a most retarded value when the hydraulic pressure control valve is in stoppage, and is supplied with hydraulic pressure for operation to advance the cam phase, and the target cam phase-restricting means sets the fixed value to the most retarded value.
According to this preferred embodiment, in accordance with setting of the target cam phase to the most retarded value, the hydraulic pressure control valve is held in a mechanically-stable stoppage state in which no hydraulic pressure is supplied thereto. This makes it possible to hold the actual cam phase in a most stable state. As a result, the variation in the air-fuel ratio can be further positively suppressed, which enables the degradation determination to be carried out with even higher accuracy.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.